U.S. patent application number 10/233083 was filed with the patent office on 2003-03-06 for methods for determining focus and astigmatism in charged-particle-beam microlithography.
This patent application is currently assigned to Nikon Corporation. Invention is credited to Suganuma, Wakako, Yahiro, Takehisa.
Application Number | 20030043358 10/233083 |
Document ID | / |
Family ID | 19089970 |
Filed Date | 2003-03-06 |
United States Patent
Application |
20030043358 |
Kind Code |
A1 |
Suganuma, Wakako ; et
al. |
March 6, 2003 |
Methods for determining focus and astigmatism in
charged-particle-beam microlithography
Abstract
Evaluation methods are disclosed for evaluating the
image-forming performance of charged-particle-beam microlithography
systems, especially with regard to astigmatism and focus. In an
embodiment, a subfield containing an evaluation pattern is
subdivided into multiple regions. In the various regions, the
respective line-and-space (L/S) pattern elements are oriented such
that the elements in one region extend in a direction that
intersects the direction, in the object plane of orientation of the
pattern element in another region. The evaluation pattern is
transferred lithographically to a resist film on a substrate. The
developed resist, when observed at a magnification at which
individual L/S pattern elements are not resolved, reveals a "shadow
region" having a particular profile. The profile is a function of
one or more parameters (e.g., astigmatism and focus) of
image-forming performance.
Inventors: |
Suganuma, Wakako; (Tokyo,
JP) ; Yahiro, Takehisa; (Agea-shi, JP) |
Correspondence
Address: |
KLARQUIST SPARKMAN, LLP
One World Trade Center
Suite 1600
121 S.W. Salmon Street
Portland
OR
97240
US
|
Assignee: |
Nikon Corporation
|
Family ID: |
19089970 |
Appl. No.: |
10/233083 |
Filed: |
August 30, 2002 |
Current U.S.
Class: |
355/53 |
Current CPC
Class: |
H01J 2237/21 20130101;
H01J 37/3174 20130101; G03F 7/706 20130101; G03F 7/70641 20130101;
H01J 37/304 20130101; H01J 2237/153 20130101; B82Y 10/00 20130101;
B82Y 40/00 20130101 |
Class at
Publication: |
355/53 |
International
Class: |
G03B 027/42 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 31, 2001 |
JP |
2001-263182 |
Claims
What is claimed is:
1. In a microlithography method in which a device pattern to be
transferred to a photosensitive substrate is defined on a reticle
situated at a reticle plane, and a region of the reticle is
illuminated with an illumination beam to form a patterned beam that
carries an image, via a projection-optical system, of the
illuminated region to the photosensitive substrate so as to form an
image of the illuminated region on the photosensitive substrate, a
method for determining an image-forming performance of the
projection-optical system, comprising: disposing an evaluation
pattern at the reticle plane, the evaluation pattern comprising
multiple groups of line-and-space (L/S) pattern elements, each
group comprising multiple L/S pattern elements extending in a
respective direction that intersects a respective direction, in the
reticle plane, of L/S pattern elements in another of the groups;
lithographically imaging the evaluation pattern on a resist-coated
substrate; developing the resist on the substrate to form in the
resist an imprinted image of the evaluation pattern; observing a
shadow region in the imprinted image of the evaluation pattern; and
from an observed profile of the shadow region, determining the
image-forming performance.
2. The method of claim 1, wherein the image-forming performance is
focus or astigmatism, or both.
3. The method of claim 1, wherein the L/S pattern elements have a
linewidth at or near a resolution limit of the projection-optical
system
4. The method of claim 1, wherein: the evaluation pattern is
divided into multiple regions; each region comprises a respective
group of L/S pattern elements; and the respective L/S pattern
elements in each of at least two regions are oriented differently
from each other in respective directions that intersect each other
in the reticle plane.
5. The method of claim 4, wherein the respective L/S pattern
elements in each of at least two regions are oriented
perpendicularly to each other.
6. The method of claim 4, wherein the respective L/S pattern
elements in each region are oriented parallel to each other.
7. The method of claim 1, wherein: the evaluation pattern is
divided into multiple regions each comprising a respective group of
L/S pattern elements; and in each group of L/S pattern elements,
the constituent L/S pattern elements extend radially relative to a
center of the subfield.
8. The method of claim 1, wherein: the evaluation pattern is
divided into multiple regions each comprising a respective group of
L/S pattern elements; and in each group of L/S pattern elements,
the constituent L/S pattern elements extend circumferentially
relative to a center of the subfield.
9. The method of claim 1, wherein the observing step is performed
using an optical microscope.
10. The method of claim 1, wherein the shadow regions arise from a
gradual increase or decrease in a linewidth of the as-imaged L/S
pattern due to a proximity effect caused by different cumulative
exposure energies from a center of the imprinted image to a
perimeter of the imprinted image of the evaluation pattern.
11. The method of claim 1, wherein: the shadow region has at least
one arc-shaped profile having a respective radius; and the radius
decreases with increasing resolution with which the evaluation
pattern is imaged lithographically from the reticle to the
substrate, resulting in a positional shift of the shadow region,
with increasing resolution, toward a center of the imprinted image
of the evaluation pattern.
12. The method of claim 11, wherein: the multiple groups of L/S
pattern elements are disposed around a center of the evaluation
pattern; and the image-forming performance pertains to astigmatism
that is determined by observing an increased radius of a respective
arc-shaped profile in a first location of the imprinted evaluation
pattern at which a blur direction caused by the astigmatism is
similar to a direction in which L/S pattern elements in the first
location extend, and observing a decreased radius of a respective
arc-shaped profile in a second location of the imprinted evaluation
pattern at which a blur direction caused by the astigmatism is
approximately 90.degree. to the direction in which L/S pattern
elements in the first location extend.
13. The method of claim 11, wherein: the image-forming performance
pertains to focus; the shadow region has a ring-shaped profile of
which a radius is a function of focus; and focus is determined by
observing the radius of the ring-shaped profile of the shadow
region.
14. The method of claim 13, wherein the radius is a function of the
extent to which an image-forming capacity of the resist matches a
position of the resist in an axial direction.
15. The method of claim 1, wherein the shadow region has an
arc-shaped profile having a radius that is a function of a
resolution with which the L/S pattern elements have been imaged in
at least one direction on the photosensitive substrate.
16. The method of claim 1, wherein the shadow region has an
arc-shaped profile having a radius that is a function of a blur
accompanying imaging of the L/S pattern elements in at least one
direction on the photosensitive substrate.
17. The method of claim 1, wherein: the resist is a negative
resist; the image of the evaluation pattern is being formed in the
resist at an incident dose greater than a pivotal-point dose for
the resist; and the darker the image, the lower a resolution at
which the evaluation pattern was imaged onto the substrate.
18. The method of claim 1, wherein: the resist is a positive
resist; the image of the evaluation pattern is being formed in the
resist at an incident dose less than a pivotal-point dose for the
resist; and the darker the image, the lower a resolution at which
the evaluation pattern was imaged onto the substrate.
19. In a microlithography method performed using a microlithography
apparatus in which a device pattern to be transferred to a
photosensitive substrate is defined on a reticle situated at a
reticle plane, and a region of the reticle is illuminated with an
illumination beam to form a patterned beam that carries an image,
via a projection-optical system, of the illuminated region to the
photosensitive substrate so as to form an image of the illuminated
region on the photosensitive substrate, a method for determining
and adjusting an imaging-forming performance of the
microlithography apparatus, comprising: setting a condensing power
of the projection-optical system; setting a focus with which the
image of the illuminated region is imaged on the photosensitive
substrate; setting an astigmatism with which the image of the
illuminated region is imaged on the photosensitive substrate;
disposing an evaluation pattern at the reticle plane, the
evaluation pattern comprising multiple groups of line-and-space
(L/S) pattern elements, each group comprising multiple L/S pattern
elements extending in a respective direction that intersects a
respective direction, in the reticle plane, of L/S pattern elements
in another of the groups; lithographically imaging the evaluation
pattern on a resist-coated substrate multiple times, wherein in
each time at least one of focus and astigmatism is changed, so as
to form multiple respective images of the evaluation pattern on the
photosensitive substrate at various respective settings of focus
and astigmatism; developing the resist on the substrate to form in
the resist respective imprinted images of the evaluation pattern;
observing respective shadow regions in the imprinted images of the
evaluation pattern; and from respective observed profiles of the
shadow regions, determining and selecting desired settings of focus
and astigmatism.
20. The method of claim 19, wherein the L/S pattern elements have a
linewidth at or near a resolution limit of the projection-optical
system
21. The method of claim 19, wherein the multiple groups of L/S
pattern elements are disposed around a center of the evaluation
pattern; and astigmatism that is determined by observing an
increased radius of a respective arc-shaped profile in a first
location of the imprinted evaluation pattern at which a blur
direction caused by the astigmatism is similar to a direction in
which L/S pattern elements in the first location extend, and
observing a decreased radius of a respective arc-shaped profile in
a second location of the imprinted evaluation pattern at which a
blur direction caused by the astigmatism is approximately
90.degree. to the direction in which L/S pattern elements in the
first location extend.
22. The method of claim 19, wherein: the shadow region has a
ring-shaped profile of which a radius is a function of focus; and
focus is determined by observing the radius of the ring-shaped
profile of the shadow region.
23. The method of claim 19, wherein the step of observing
respective shadow regions is performed using an optical microscope.
Description
FIELD
[0001] This disclosure pertains to microlithography, which is a key
technology used in the fabrication of micro-electronic devices such
as semiconductor integrated circuits, displays, and the like. More
specifically, the disclosure pertains, in the context of
microlithography performed using a charged particle beam, to
methods for evaluating the image-forming performance of the
charged-particle-beam (CPB) optical system as used in a CPB
microlithography system. Such image-forming evaluations include,
for example, astigmatism and focusing. The disclosure also is
directed to methods for adjusting the microlithography system based
on data obtained from the image-forming evaluations.
BACKGROUND
[0002] As the sizes of active circuit elements in micro-electronic
devices have continued to decrease, with concurrent increases in
device-packing density, the development of "next-generation"
lithography (NGL) systems and related methods has been rapid.
Currently favored approaches to NGL technology utilize very short
wavelengths of light (specifically, "extreme ultraviolet", or
"EUV", light) and charged particle beams (specifically, electron
beams and ion beams) in an effort to produce finer pattern
resolution than currently obtainable using conventional optical
microlithography.
[0003] Regarding charged-particle-beam (CPB) microlithography,
developments in electron-beam lithography have been especially
rapid. An electron beam has an excellent propensity to propagate in
a straight line, and thus is well-suited for making
microlithographic exposures of extremely fine patterns. At the time
electron-beam microlithography made its debut, patterns were
"drawn" line-by-line on the substrate using an electron beam. This
technique exhibited extremely low throughput, especially in
contrast to optical microlithography in which an entire pattern can
be exposed from the reticle to the substrate in a single "shot."
Unfortunately, electron-beam microlithography currently is
incapable of transferring an entire pattern from a reticle to a
substrate in a single shot. But, to obtain substantially better
throughput than obtained using the line-by-line drawing technique,
the pattern as defined on the reticle is divided into a large
number of portions, termed "subfields," each defining a respective
set of pattern elements. The subfields are exposed individually in
a sequential manner in respective shots onto the substrate. The
respective images of the subfields are positioned accurately on the
substrate so as to achieve proper "stitching" of the images into a
contiguous entire pattern on the substrate. This technique is
termed the "divided-reticle" transfer-exposure technique.
[0004] Divided-reticle transfer-exposure has been shown capable of
resolving 70-nm pattern elements, especially with recent
improvements in the performance of resists applied to the substrate
surface. It is anticipated that, with upcoming demands for circuit
elements having dimensions of 50 nm or less, beam astigmatism and
focal position will have significant impacts on pattern resolution.
Currently, the size of a subfield as projected onto the wafer using
an electron beam has dimensions of 250 .mu.m square. With such
dimensions a high beam-acceleration voltage is used, but the
current density per unit area is low. Under such conditions, it is
difficult to adjust astigmatism or focus using a mark-scanning
waveform as used in conventional variable-shaped-beam exposure
methods.
[0005] Hence, in conventional divided-reticle transfer-exposure,
after completing transfer-exposure of the entire pattern to the
resist layer on the surface of the substrate, the pattern actually
formed in the resist is measured to evaluate astigmatism and
resolution. Correction of astigmatism is performed using an
astigmatism-correction coil ("stigmator") situated in the
electron-beam optical system upstream of the substrate. An
exemplary conventional stigmator is shown in FIGS. 8(A)-8(B),
wherein FIG. 8(A) is a plan view of the stigmator coils, and FIG.
8(B) is a schematic diagram of the operating principles of the
coils shown in FIG. 8(A). The stigmator 100 comprises two sets of
quadrupole coils A and B. The two sets of quadrupole coils are
disposed so that the coils A-1 to A-4 in the set A and the coils
B-1 to B-4 in the set B are positioned altematingly. The respective
axis of each coil is oriented radially and at a right angle to the
optical axis (the Z-axis), and the individual coil axes are
oriented 45.degree. apart from each other. The two sets A, B of
coils are connected to and driven by respective power supplies
103a, 103b, respectively. Note that the coils of each set are
connected together in series to the respective power supply.
[0006] Operation of the stigmator 100 is described with reference
now to FIG. 8(B), depicting only the coils A-1 to A-4 of the first
set A of quadrupole coils. As electrical current flows from the
respective power supply to the four coils of the set, the coils
generate respective magnetic fields (indicated by respective lines
of force). An electron beam traveling in the Z-direction
(perpendicular to the plane of the page) near the optical axis is
influenced by the magnetic fields and is laterally deflected
(according to Fleming's left-hand rule) in the directions indicated
by the white arrows. Thus, the portion of an electron beam of which
the transverse section extends diagonally from the upper right
corner to the lower left corner in the figure is urged, by
deflection, toward the optical axis. Similarly, the portion of the
beam of which the transverse section extends diagonally from the
upper left corner to the lower right corner in the figure is urged,
by deflection, away from the optical axis. As a result, the aerial
image carried by the electron beam passing in a generally axial
direction through the stigmator of FIG. 8(B) is compressed in a
first lateral direction and expanded in a second lateral direction
orthogonal to the first lateral direction. These compressions and
expansions cancel astigmatism extending in directions opposite the
directions of compression and expansion. A similar expansion and
contraction action is achieved by the other set B of coils. By
controlling the current supplied from the respective power supplies
to the coil sets, astigmatism in the various directions is
corrected.
[0007] For establishing optimal astigmatism adjustments to be made
by the stigmator, a layer of resist on a downstream substrate is
exposed under conditions in which the current supplied to the
stigmator coils is varied according to a predetermined pitch. The
resulting patterns formed in the exposed resist are observed under
a scanning electron microscope (SEM) at a magnification of
10,000.times. or more. Based on the results of this observation,
the stigmator is adjusted so as to produce optimal resolution.
[0008] To correct focus, electrical current supplied to a
focus-adjustment coil in the electron-beam optical system is varied
while exposing a pattern multiple times under respective conditions
in which the convergence point for the beam is moved axially "up"
and "down." The resulting images are observed by SEM to determine
an adjustment to the focus-adjustment coil appropriate for
achieving optimal resolution.
[0009] Evaluation patterns used for adjustments to astigmatism and
focus normally are patterns in which constituent line-and-space
(L&S) elements are disposed "vertically" and "horizontally"
(i.e., in X and Y directions) in subfields of a reticle. A line
element of such a pattern can have multiple widths ranging from the
resolution-limited linewidth to several times the
resolution-limited linewidth. For performing the evaluation, first
a reticle is used on which the evaluation pattern is formed in a
large number of subfields. As each subfield is exposed, three
parameters (the current supplied to the focus-adjustment coil and
the respective currents supplied to the two coil sets of the
stigmator) are varied stepwise. After completing exposure, the
wafer is developed, and the pattern thus formed is observed by SEM.
The subfield pattern having the best resolution is determined, and
the respective combination of parameters used for exposing that
subfield is selected as optimal exposure conditions.
[0010] Determining optimal conditions for correcting astigmatism
and focus as described above requires exposing a substrate each
instance in which the current supplied to the focus-adjustment coil
and the respective currents supplied to the two coil sets in the
stigmator are varied, and observing the developed pattern by SEM.
These operations must be performed on a number (tens or hundreds)
of subfields equal to the number of parameters multiplied by the
number of conditions involved. As a result, these operations are
very time-consuming. In addition, evaluation of focus and
astigmatism by SEM requires considerable experience to perform in a
manner yielding useful data.
SUMMARY
[0011] In view of the shortcomings of the prior art as summarized
above, the present invention provides, inter alia, improved methods
for evaluating the image-forming performance (e.g., astigmatism
and/or focus) of a charged-particle-beam (CPB) microlithography
system, wherein the evaluation and adjustment of the imageforming
performance can be performed simply and efficiently.
[0012] One aspect of the invention is directed to methods set forth
in the context of a microlithography method in which a device
pattern to be transferred to a photosensitive substrate is defined
on a reticle situated at a reticle plane. A region of the reticle
is illuminated with an illumination beam to form a patterned beam
that carries an image, via a projection-optical system, of the
illuminated region to the photosensitive substrate so as to form an
image of the illuminated region on the photosensitive substrate.
The subject methods are for determining the imageforming
performance of the projection-optical system. In an embodiment of
such a method, an evaluation pattern is disposed at the reticle
plane. The evaluation pattern comprises multiple groups of
line-and-space (L/S) pattern elements, each group comprising
multiple L/S pattern elements extending in a respective direction
that intersects a respective direction, in the reticle plane, of
L/S pattern elements in another of the groups. The evaluation
pattern is lithographically imaged on a resist-coated substrate,
and the resist on the substrate is developed to form in the resist
an imprinted image of the evaluation pattern. Observations are made
of a "shadow region" (as described herein) in the imprinted image
of the evaluation pattern. From the observed profile of the shadow
region, the image-forming performance is determined. The
image-forming performance can be focus or astigmatism, or both.
[0013] Desirably, the L/S pattern elements have a linewidth at or
near a resolution limit of the projection-optical system.
[0014] The evaluation pattern can be divided into multiple regions,
wherein each region comprises a respective group of L/S pattern
elements. The respective L/S pattern elements in each of at least
two regions are oriented differently from each other in respective
directions that intersect each other in the reticle plane. For
example, the respective L/S pattern elements in each of at least
two regions can be oriented perpendicularly to each other. In each
group, the constituent L/S pattern elements can be parallel to each
other. Alternatively, in each of multiple regions each comprising a
respective group of L/S pattern elements, the constituent L/S
pattern elements can extend radially relative to a center of the
subfield. Further alternatively, in each of multiple regions each
comprising a respective group of L/S pattern elements, the
constituent L/S pattern elements can extend circumferentially
relative to a center of the subfield.
[0015] The evaluation pattern can be defined on a reticle subfield
divided into multiple regions. In this configuration, each region
comprises a respective group of L/S pattern elements. The
respective L/S pattern elements in each group are disposed around a
center of the evaluation pattern.
[0016] In this method embodiment, the observing step desirably is
performed using an optical microscope, which allows easy and rapid
observations with minimal operator training.
[0017] The shadow regions arise generally from a gradual increase
or decrease in a linewidth of the as-imaged L/S pattern due to a
proximity effect caused by different cumulative exposure energies
from a center of the imprinted image to a perimeter of the
imprinted image of the evaluation pattern. The shadow regions can
have any of several profiles. For example, the shadow region can
have at least one arc-shaped profile having a respective radius,
wherein the radius decreases with increasing resolution with which
the evaluation pattern is lithographically imaged from the reticle
to the substrate, resulting in a positional shift of the shadow
region, with increasing resolution, toward a center of the
imprinted image of the evaluation pattern. In this embodiment the
multiple groups of L/S pattern elements can be disposed around a
center of the evaluation pattern. If the image-forming performance
pertains to astigmatism, then astigmatism is determined by
observing an increased radius of a respective arc-shaped profile in
a first location of the imprinted evaluation pattern at which a
blur direction caused by the astigmatism is similar to a direction
in which L/S pattern elements in the first location extend, and
observing a decreased radius of a respective arc-shaped profile in
a second location of the imprinted evaluation pattern at which a
blur direction caused by the astigmatism is approximately
90.degree. to the direction in which L/S pattern elements in the
first location extend. If the image-forming performance pertains to
focus, then the shadow region can have a ring-shaped profile of
which a radius is a function of focus, wherein focus is determined
by observing the radius of the ring-shaped profile of the shadow
region. The radius can be a function of the extent to which an
image-forming capacity of the resist matches a position of the
resist in an axial direction.
[0018] The shadow region can have an arc-shaped profile having a
radius that is a function of a resolution with which the L/S
pattern elements have been imaged in at least one direction on the
photosensitive substrate. Alternatively or in addition, the shadow
region can have an arc-shaped profile having a radius that is a
function of a blur accompanying imaging of the L/S pattern elements
in at least one direction on the photosensitive substrate.
[0019] If the resist is a negative resist and the incident dose is
greater than a "pivotal-point" dose, then the darker the image, the
lower a resolution at which the evaluation pattern was imaged onto
the substrate. On the contrary, if the incident dose is less than
the pivotal-point dose, then the darker the image, the higher a
resolution at which the evaluation pattern was imaged onto the
substrate. The "pivotal-point dose" is the incident dose at which
line elements develop in an exposed negative resist (or spaces
develop in an exposed positive resist) with no changes in linewidth
occurring with changes in blur. Actual pivotal-point doses are
established by properties of the resist, and may vary from one type
or brand of resist to another. If the resist is a positive resist
and the incident dose is less than the pivotal-point dose, then the
darker the image, the lower a resolution at which the evaluation
pattern was imaged onto the substrate.
[0020] Another aspect of the invention also is set forth in the
context of a microlithography method performed using a
microlithography apparatus in which a device pattern to be
transferred to a photosensitive substrate is defined on a reticle
situated at a reticle plane. A region of the reticle is illuminated
with an illumination beam to form a patterned beam that carries an
image, via a projection-optical system, of the illuminated region
to the photosensitive substrate so as to form an image of the
illuminated region on the photosensitive substrate. The subject
method of this aspect is directed to determining and adjusting an
imaging-forming performance of the microlithography apparatus. An
embodiment of such a method comprises setting a condensing power of
the projection-optical system, setting a focus with which the image
of the illuminated region is imaged on the photosensitive
substrate, and setting an astigmatism with which the image of the
illuminated region is imaged on the photosensitive substrate. Also,
an evaluation pattern is disposed at the reticle plane. The
evaluation pattern comprises multiple groups of L/S pattern
elements, wherein each group comprises multiple L/S pattern
elements extending in a respective direction that intersects a
respective direction, in the reticle plane, of L/S pattern elements
in another of the groups. The evaluation pattern is
lithographically imaged on a resist-coated substrate multiple
times, wherein in each time at least one of focus and astigmatism
is changed, so as to form. multiple respective images of the
evaluation pattern on the photosensitive substrate at various
respective settings of focus and astigmatism. The resist on the
substrate is developed to form in the resist respective imprinted
images of the evaluation pattern. Respective shadow regions in the
imprinted images of the evaluation pattern are observed. From
respective observed profiles of the shadow regions, desired
settings of focus and astigmatism are determined and selected.
[0021] As noted above, the L/S pattern elements desirably have a
linewidth at or near a resolution limit of the projection-optical
system
[0022] The multiple groups of L/S pattern elements desirably are
disposed around a center of the evaluation pattern. With such a
configuration of the evaluation pattern, astigmatism can be
determined by observing an increased radius of a respective
arc-shaped profile in a first location of the imprinted evaluation
pattern at which a blur direction caused by the astigmatism is
similar to a direction in which L/S pattern elements in the first
location extend, and observing a decreased radius of a respective
arc-shaped profile in a second location of the imprinted evaluation
pattern at which a blur direction caused by the astigmatism is
approximately 90.degree. to the direction in which L/S pattern
elements in the first location extend.
[0023] The shadow region can have a ring-shaped profile of which a
radius is a function of focus, wherein focus is determined by
observing the radius of the ring-shaped profile of the shadow
region.
[0024] The foregoing and additional features and advantages of the
invention will be more readily apparent from the following detailed
description, which proceeds with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIGS. 1(A)-1(B) are plan views of respective
astigmatism-evaluation patterns as described in the first
representative embodiment, and of corresponding dark regions,
observable using a low-power optical microscope, formed when the
patterns are exposed onto a negative-resist-coated wafer under
certain astigmatism conditions.
[0026] FIG. 2 is an elevational schematic diagram showing the
overall configuration of an embodiment of a charged-particle-beam
microlithography system of which the optical system is evaluated
using an astigmatism-evaluation pattern according to, e.g., any of
the representative embodiments described herein.
[0027] FIG. 3 is a plan view of an astigmatism-evaluation pattern
as described in the second representative embodiment, and of
corresponding dark regions, observable using a low-power optical
microscope, formed when the pattern is exposed onto a
negative-resist-coated wafer under certain astigmatism
conditions.
[0028] FIG. 4 is a two-dimensional matrix of various shapes of dark
regions that are formed on a negative resist from the evaluation
pattern of the first representative embodiment, under respective
conditions of relative energization of two coil sets in a stigmator
such as that shown in FIG. 8(A).
[0029] FIG. 5 is a one-dimensional matrix of various shapes of dark
regions that are formed on a negative resist from an exemplary
evaluation pattern as the focal point of the pattern image is
changed.
[0030] FIG. 6 is a plan view of an astigmatism-evaluation pattern
according to the third representative embodiment.
[0031] FIG. 7 is a plan view of an alternative
astigmatism-evaluation pattern according to the third
representative embodiment.
[0032] FIGS. 8(A)-8(B) depict the configuration of an exemplary
stigmator, wherein FIG. 8(A) is a plan view showing both sets of
coils of the stigmator, and FIG. 8(B) is a plan view showing
certain operational principles of the coil set "A" shown in FIG.
8(A).
[0033] FIGS. 9(A)-9(B) are respective plots of certain parameters
affecting the configuration and location of dark regions on a
negative resist exposed with an astigmatism-evaluation pattern,
wherein FIG. 9(A) is a plot of the distribution of mean residual
resist thickness in a subfield, and FIG. 9(B) is a plot of the
distribution of exposure dose in a subfield.
DETAILED DESCRIPTION
[0034] The invention is described below in the context of
representative embodiments, which are not to be regarded as
limiting in any way.
[0035] FIG. 2 shows an overview of a CPB divided-reticle
projection-microlithography system. The depicted system utilizes an
electron beam as an exemplary charged particle beam. Situated at
the extreme upstream end of the system is an electron gun 1 that
emits an electron beam propagating in a downstream direction
generally along an optical axis Ax. Downstream of the electron gun
1 are a first condenser lens 2 and a second condenser lens 3
collectively constituting a two-stage condenser-lens assembly. The
condenser lenses 2, 3 converge the electron beam at a crossover
C.O. situated on the optical axis Ax at a blanking diaphragm 7.
[0036] Downstream of the second condenser lens 3 is a "beam-shaping
diaphragm" 4 comprising a plate defining an axial aperture
(typically rectangular in profile) that trims and shapes the
electron beam passing through the aperture. The aperture is sized
and configured to trim the electron beam sufficiently to illuminate
one subfield on the divided reticle 10 per shot. An image of the
beam-shaping diaphragm 4 is formed on the reticle 10 by an
illumination lens 9.
[0037] The electron-optical components situated between the
electron gun 1 and the reticle 10 collectively constitute an
"illumination-optical system" of the depicted microlithography
system. The electron beam propagating through the
illumination-optical system is termed an "illumination beam"
because it illuminates a desired region of the reticle 10. As the
illumination beam propagates through the illumination-optical
system, the beam actually travels in a downstream direction through
an axially aligned "beam tube" (not shown but well understood in
the art) that can be evacuated to a desired vacuum level.
[0038] A blanking deflector 5 is situated downstream of the
beam-shaping aperture 4. The blanking deflector 5 laterally
deflects the illumination beam as required to cause the
illumination beam to strike the aperture plate of the blanking
diaphragm 7, thereby preventing the illumination beam from being
incident on the reticle 10.
[0039] A subfield-selection deflector 8 is situated downstream of
the blanking diaphragm 7. The subfield-selection deflector 8
laterally deflects the illumination beam as required to illuminate
a desired subfield on the reticle within the optical field of the
illumination optical system. Thus, the subfields of the reticle 10
are scanned sequentially by the illumination beam in a horizontal
direction (X direction in the figure). The illumination lens 9,
which forms the image of the beam-shaping diaphragm 4 on the
reticle 10, is situated downstream of the subfield-selection
deflector 8.
[0040] The divided reticle 10 typically defines many subfields
(e.g., tens of thousands of subfields) and may be manufactured
using any of the methods discussed below. The subfields
collectively define the pattern for a layer to be formed at a
single die ("chip") on a lithographic substrate. The reticle 10 is
mounted on a movable reticle stage 11. Using the reticle stage 11,
by moving the reticle 10 in a direction (Y and/or X direction)
perpendicular to the optical axis Ax, it is possible to illuminate
the respective subfields on the reticle 10 extending over a range
that is wider than the optical field of the illumination-optical
system. The position of the reticle stage 11 in the XY plane is
determined using a "position detector" 12 typically configured as a
laser interferometer. The laser interferometer is capable of
measuring the position of the reticle stage 11 with extremely high
accuracy in real time.
[0041] Situated downstream of the reticle 10 are first and second
projection lenses 15, 19, respectively. The illumination beam, by
passage through an illuminated subfield of the reticle IO, becomes
a "patterned beam" because the beam has acquired an aerial image of
the illuminated subfield. The patterned beam is imaged at a
specified location on a substrate 23 (e.g., "wafer") by the
projection lenses 15, 19 collectively functioning as a
"projection-lens assembly."
[0042] So as to be imprintable with the image carried by the
patterned beam, the upstream-facing surface of the substrate 23 is
coated with a suitable "resist" that is imprintably sensitive to
exposure by the patterned beam. A resist-coated substrate is termed
herein "photosensitive," whether the resist is sensitive to
exposure by electromagnetic radiation or a charged particle beam.
When forming the image on the substrate, the projection-lens
assembly "reduces" (demagnifies) the aerial image. Thus, the image
as formed on the substrate 23 is smaller (usually by a defined
integer-ratio factor termed the "demagnification factor," such as
1/4) than the corresponding region illuminated on the reticle 10.
By thus causing imprinting on the surface of the substrate 23, the
apparatus of FIG. 2 achieves "transfer" of the pattern image from
the reticle 10 to the substrate 23.
[0043] The components of the depicted electron-optical system
situated between the reticle 10 and the substrate 23 collectively
are termed the "projection-optical system." The substrate 23 is
situated on a substrate stage 24 situated downstream of the
projection-optical system. As the patterned beam propagates through
the projection-optical system, the beam actually travels in a
downstream direction through an axially aligned "beam tube" (not
shown but well understood in the art) that can be evacuated to a
desired vacuum level.
[0044] The projection-optical system forms a crossover C.O. of the
patterned beam on the optical axis Ax at the rear focal plane of
the first projection lens 15. The position of the crossover C.O. on
the optical axis Ax is a point at which the axial distance between
the reticle 10 and substrate 23 is divided according to the
demagnification factor. Situated between the crossover C.O. (i.e.,
the rear focal plane) and the reticle 10 is a contrast-aperture
diaphragm 18. The contrast-aperture diaphragm 18 comprises an
aperture plate that defines an aperture. With the contrast-aperture
diaphragm 18, electrons of the patterned beam that were scattered
during transmission through the reticle 10 are blocked so as not to
reach the substrate 23.
[0045] A backscattered-electron (BSE) detector 22 is situated
immediately upstream of the substrate 23. The BSE detector 22 is
configured to detect and quantify electrons backscattered from
certain marks situated on the upstream-facing surface of the
substrate 23 or on an upstream-facing surface of the substrate
stage 24. For example, a mark on the substrate 23 can be scanned by
a beam that has passed through a corresponding mark pattern on the
reticle 10. By detecting backscattered electrons from the mark at
the substrate 23, it is possible to determine the relative
positional relationship of the reticle 10 and the substrate 23.
[0046] The substrate 23 is mounted to the substrate stage 24 via a
wafer chuck (not shown but well understood in the art), which
presents the upstream-facing surface of the substrate 23 in an XY
plane. The substrate stage 24 (with chuck and substrate 23) is
movable in the X and Y directions. Thus, by simultaneously scanning
the reticle stage 11 and the substrate stage 24 in mutually
opposite directions, it is possible to transfer-expose each
subfield within the optical field of the illumination-optical
system as well as each subfield outside the optical field to
corresponding regions on the substrate 23. The substrate stage 24
also includes a "position detector" 25 configured similarly to the
position detector 12 of the reticle stage 11.
[0047] Each of the lenses 2, 3, 9, 15, 19 and deflectors 5, 8 is
controlled by a controller 31 via a respective coil-power
controller 2a, 3a, 9a, 15a, 19a and 5a, 8a. Similarly, the reticle
stage 11 and substrate stage 24 are controlled by the controller 31
via respective stage drivers 11a, 24a. The position detectors 12,
25 produce and route respective stage-position signals to the
controller 31 via respective interfaces 12a, 25a each including
amplifiers, analog-to-digital (A/D) converters, and other circuitry
for achieving such ends. In addition, the BSE detector 22 produces
and routes signals to the controller 31 via a respective interface
22a.
[0048] From the respective data routed to it, the controller 31
ascertains, inter alia, any control errors of the respective stage
positions as a subfield is being transferred, and actuates
appropriate control or restorative measures as required. Thus, a
reduced image of the illuminated subfield on the reticle 10 is
transferred accurately to the desired target position on the
substrate 23. This real-time correction is made as each respective
image of a subfield is transferred to the substrate 23, and the
subfield images are positioned such that they are stitched together
properly on the substrate 23.
[0049] Astigmatism of a system such as that shown in FIG. 2 is
evaluated using an astigmatism-evaluation pattern 50 according to a
first representative embodiment, as shown in FIGS. 1(A)-1(B). The
evaluation pattern 50 in this embodiment includes a large number of
thin, parallel "light" and "dark" elements termed "line-and-space"
(L/S) elements. The evaluation pattern 50 is created by splitting a
subfield 51 into two subunits in the X direction and two subunits
in the Y direction, thereby forming four rectangular regions S1,
S2, S3, S4. Each region S1-S4 defines a large number of respective
L/S elements. By way of example, the subfield 51 has edge
dimensions of 1 mm on the reticle, and 0.25 mm as projected onto
the substrate (wafer). The individual lines and spaces have equal
linewidths, wherein each individual linewidth is near the
resolution limit of the electron-beam microlithography system
(e.g., 70 nm on the wafer). In the evaluation pattern 50, the L/S
elements extend in the X direction in the regions S2 (in the upper
right in the figure) and S4 (in the lower left in the figure). The
L/S elements extend in the Y direction in the regions S1 (in the
upper left in the figure) and S3 (in the lower right in the
figure).
[0050] In an example, the evaluation pattern 50 was
transfer-exposed onto a resistcoated wafer using an electron-beam
microlithography apparatus such as that shown in FIG. 2.
Surprisingly, the images as formed on the wafer revealed useful
features that could be observed and evaluated using a simple
optical microscope rather than a complex SEM. In this example, the
resulting resist pattern as formed on the wafer was observed using
a 100.times.optical microscope employing white light for
illumination. The resist pattern revealed dark "shadow" regions Fo
or Fi (FIGS. 1(A)-1(B)). Although shown in FIGS. 1(A)-1(B),
individual lines and spaces of the evaluation pattern were not
resolvable using the optical microscope due to their being below
the resolution limit of the microscope. The microscope image was
displayed on a monitor using an ITV camera with adjusted contrast.
The "shadow" regions Fo, Fi are peculiar to a negative-resist image
of an L/S grid in which pattern-element resolution is relatively
adequate. The shadow regions Fo, Fi arise from the distribution of
cumulative electron dose from the center of the subfield toward the
periphery, and from an increased linewidth near the center of the
subfield. This condition creates small arcuate shadow regions Fi
located in the center of the subfield (the greater the resolution,
the smaller the radius of the arcuate shadow regions). As
resolution decreases, the arcs shift toward the periphery of the
subfield (shadow regions Fo in the figure). Further decreases in
resolution cause the entire image of the evaluation pattern to
appear dark (in a negative resist).
[0051] In FIGS. 1(A)-1(B), the arc-shaped shadow regions Fo, Fi
appear differently depending upon the direction of the L/S pattern,
due to the influence of astigmatism on direction of blur. If
astigmatism is present that causes the image to extend in a certain
X-Y direction that is substantially the same as the longitudinal
direction of the L/S pattern, then the linewidth essentially is
unchanged by the astigmatism and the resolution remains adequate.
However, if the astigmatism is 90.degree. to the longitudinal
direction of the L/S pattern, then the astigmatism causes an
increase in the linewidth of the projected pattern, with a
corresponding decrease in resolution.
[0052] Hence, FIG. 1(A) depicts a situation in which astigmatism
extending in the Y direction is present. This astigmatism increases
the linewidth (as projected) of L/S lines extending in the X
direction, while L/S lines extending in the Y direction essentially
are unaffected by the astigmatism. Consequently, the resolution of
individual L/S pattern elements in the upper right region S2 and
the lower left region S4 is reduced, while the resolution of
individual L/S pattern elements in the upper left region S1 and the
lower right region S3 is unchanged. As indicated in the figure, in
the upper right and lower left regions S2 and S4, respectively, the
arcs of the respective shadow regions Fo are situated near the
periphery of the subfield 51 (where image "lightness" is least). In
the upper left and lower right regions S1 and S3, respectively, the
arcs of the respective shadow regions Fi are situated near the
center of the subfield.
[0053] FIG. 1(B) depicts a situation in which astigmatism extending
in the X direction is present. This astigmatism increases the
linewidth (as projected) of L/S lines extending in the Y direction,
while L/S lines extending in the X direction essentially are
unaffected by the astigmatism. Consequently, the resolution of
individual L/S pattern elements in the upper left region S1 and the
lower right region S3 is reduced, while the resolution of
individual L/S pattern elements in the upper right region S2 and
the lower left region S4 is unchanged. As indicated in the figure,
in the upper left and lower right regions S1 and S3, respectively,
the arcs of the respective shadow regions Fo are situated near the
periphery of the subfield 51 (where image "lightness" is least). In
the upper right and lower left regions S2 and S4, respectively, the
arcs of the respective shadow regions Fi are situated near the
center of the subfield.
[0054] The mechanism by which the shadow regions occur is shown
schematically in FIG. 9(A), depicting a distribution of mean
residual resist thickness in a subfield. FIG. 9(B) depicts a
corresponding distribution of exposure dose in a subfield as
projected onto the wafer. The abscissa in FIGS. 9(A) and 9(B) is
position within a subfield (wherein "0" is the center of the
subfield). The ordinate in FIG. 9(A) is mean thickness of residual
resist. The ordinate in FIG. 9(B) is exposure dose.
[0055] The occurrence of a shadow region appears to be mediated by
the thickness of the residual resist on the wafer and by the
spectrum of illumination light used with the optical microscope for
observing the images of the evaluation pattern, according to the
following relationship. If d denotes the mean thickness of the
residual resist on the wafer, .lambda. denotes the mean wavelength
of the illumination light, and n denotes the refractive index, then
d=1/2n.multidot.(N+1/2).m- ultidot..lambda. (wherein N is an
integer, and N-0 in this case). The threshold mean thickness of
resist in which a shadow region occurs is indicated by "Z" in FIG.
9(A). The occurrence of a shadow region is affected by the spectrum
of light-sensitivity for the human eye (or by the photosensitive
spectrum for CCD or ITV cameras).
[0056] The shadow regions Fi in the upper left and lower right
regions in FIG. 1(A) and in the upper right and lower left regions
in FIG. 1(B) have an arcuate shape near the center of the subfield
51. Because the direction of the L/S pattern and the direction of
minimal blur due to astigmatism are identical in these regions, the
shadow region is unaffected by the aberration. Therefore, the
occurrence of the shadow regions in this case appears to be caused
by a proximity effect arising whenever the mean residual film
thickness near the center of the subfield is at or above a
prescribed thickness ("Z"). In such an instance, in the plane of
FIG. 9(A), shadow regions appear at locations displaced a distance
D1 from the center of the subfield. At D1, the image of the shadow
region is darkest (with a negative resist). Further with respect to
a negative resist, the image is relatively dark from D1 toward the
center of the subfield, and lightens from D1 further toward the
periphery of the subfield.
[0057] The reason shadow regions appear at the center of a subfield
appears to be due to the manner in which the proximity effect
manifests itself. In FIG. 9(B), in the absence of the proximity
effect, the exposure dose is distributed constantly in the
subfield, as indicated by the dotted line. On the other hand, in
the presence of the proximity effect, the distribution of exposure
dose exhibits a peak at the center of the subfield, as indicated by
the solid-line curve.
[0058] As described above, the exposure dose in a subfield exhibits
a distribution due to the proximity effect. The mean residual film
thickness increases near the center of the subfield according to
this distribution. Thus, the mean residual film thickness is
distributed so as to have a peak located near the center of the
subfield in the plane of the subfield. The thickness decreases
outwardly from the center in a radial direction.
[0059] The shadow regions Fo appearing in the upper right and lower
left regions in FIG. 1(A) and in the upper left and lower right
regions in FIG. 1(B) are located near the periphery of the subfield
51. In these shadow regions, the direction of the L/S pattern
elements and the direction of blur caused by the astigmatism
intersect each other orthogonally. This causes the mean residual
film thickness to increase. The combined influence of the proximity
effect and the aberration causes an increase in mean residual film
thickness, which causes the distribution of mean residual thickness
in the subfield to shift upward. In a negative resist, the dark
regions appear to be produced by interference as the mean residual
film thickness reaches the prescribed thickness Z in subfield
regions located outwardly from the position D1.
[0060] The greater the aberration, the greater the tendency of the
arcuate shadow regions to be formed near the periphery of the
subfield. Similarly, the greater the aberration, the greater the
extent to which the mean residual film thickness is incremented.
Whenever the increment, created by the proximity effect, is added
to the distribution of mean residual film thickness having a peak
in the center of the subfield, the distribution exhibits a gradual
upward shift. As the increment due to aberration increases, the
position at which the mean residual film thickness is Z is shifted
outward from the center (0) according to the distribution described
above, which causes the shadow regions to be formed toward the
periphery of the subfield.
[0061] In the absence of any aberration and whenever the resolution
of L/S pattern elements on the resist is adequate, the substrate
side of the resist is exposed to the spaces, which causes the
pattern image to become lighter. On the other hand, if resolution
is degraded due to an astigmatism, the lines overlap and the resist
material tends to remain, which causes the entire pattern (as
imaged on a negative resist) to become darker. This phenomenon as
described occurs with a negative resist. In the case of a positive
resist, spaces in the L/S pattern are exposed rather than lines
being exposed with a negative resist. Hence, with a positive resist
the pattern image becomes lighter as resolution is degraded.
[0062] Therefore, with a negative resist the greater the
resolution, the lighter the pattern, and the smaller the radius of
the dark region. As resolution is degraded, the imaged pattern
becomes darker, and the radius of the dark region increases toward
the periphery of the subfield. The overall dark image suppresses
the occurrence of shadow regions.
[0063] Another apparent reason for the occurrence of arc-shaped
shadow regions is a change in line (or space) width of the L/S
pattern caused by the proximity effect or by astigmatism. The width
change results in a diffraction effect, which forms the shadow
regions whenever the pattern is observed under an optical
microscope.
[0064] FIG. 3 depicts the orientation of L/S elements in the
evaluation pattern and the resulting shadow regions as observed in
a second representative embodiment. The evaluation pattern 60,
similar to the evaluation pattern 50 of FIGS. 1(A)-1(B), comprises
four rectangular regions S1, S2, S3, S4 in the subfield 61, each
region containing a respective large number of L/S pattern
elements. In this pattern, the L/S elements extend in the X
direction in the upper right region S2 of the subfield; extend in
the Y direction in the lower right region S3, extend in a
-45.degree. direction in the upper left region S1, and in a
+45.degree. direction in the lower left region S4.
[0065] Observation of the resist pattern to which this evaluation
pattern 60 is transferred desirably is performed using an optical
microscope as described above. In the presence of astigmatism
extending in the Y direction, the shadow region Fo extends along
the periphery of the upper right region S2, similar to what is
shown in FIG. 1(A). In the lower right region S3, the shadow region
Fi is situated near the center of the subfield. In the upper left
and lower left regions S1 and S4, respectively, the lines extending
diagonally are affected equally by an equivalent astigmatism. As a
result, a shadow region Fm occurs at a radial distance mid-way
between the peripheral shadow region Fo and the central shadow
region Fi.
[0066] FIG. 4 shows various shadow-region patterns formed on a
resist exposed while changing the supply-current parameters for the
two coil sets in the stigmator. The evaluation pattern used in this
case is as shown in FIG. 1(A).
[0067] Using a stigmator as shown in FIG. 8(A), the current
supplied to the set of coils A-1 to A-4 is increased in five steps
from A1 to A5 in FIG. 4. The current supplied to the set of coils
B-1 to B-4 similarly is increased in five steps from B1 to B5 in
FIG. 4. Thus, respective subfields are exposed in 5.times.5=25
different conditions representing respective permutations of the
respective currents supplied to each set of coils of the stigmator.
For the "A" set of coils aberration is adjusted to cause the image
to extend in a 45.degree. to 135.degree. direction; for the "B" set
of coils aberration is adjusted to cause the image to extend in the
X and Y directions. The optical microscope used for pattern
observations is as described above.
[0068] As indicated in FIG. 4, increasing the current supplied to
the "A" coils and the "B" coils causes corresponding changes in the
shapes of the shadow regions that are produced in the four regions
of the imaged evaluation pattern. The changes are due to changes in
the direction and magnitude of the aberration to be corrected (due
to the geometry of the coils in the astigmatism-correction coils of
the stigmator) and to the respective currents supplied to the
coils.
[0069] In this example, whenever the current supplied to the "A"
coils is A3 and the current supplied to the "B" coils is B3 (i.e.,
at the center of the figure), the resulting shadow region appear
collectively as a single ring having a relatively small diameter at
the center of the subfield image. These conditions are interpreted
as producing the smallest aberration. Thus, by varying the current
supplied to the coils in the stigmator, conditions producing the
smallest-diameter dark ring and the greatest "lightness" are
determined.
[0070] These observations can be performed very efficiently using
an optical microscope.
[0071] FIG. 5 depicts changes in the profile of shadow regions
observed with corresponding changes in the focal position of an
imaged evaluation pattern. The focal position is varied by changing
the current supplied to the projection lenses 15, 19 in the system
shown in FIG. 2. The figure illustrates cases in which no
astigmatism is present. The shadow region F appears as a single
spot or contiguous ring in the subfield, depending upon the focal
position of the image.
[0072] Changing focus under a condition in which no astigmatism is
present causes shadow regions to be formed solely by the proximity
effect. Whenever the image is at best focus (part "a" of FIG. 5),
the shadow region F is situated in the center of the subfield and
appears as a spot. Because resolution is favorable under this
condition, the entire evaluation-pattern image, as projected onto a
negative resist, is light. As focus is varied progressively away
from best focus, the shadow region F expands to a ring shape that
progressively increases in diameter, due to a proximity effect, to
near the periphery of the subfield image, according to a
corresponding distribution of linewidth increments (parts "b"-"d"
of FIG. 5). Ultimately (part "e" of FIG. 5), no distinct shadow
regions are evident as the entire image darkens.
[0073] Therefore, as the respective currents supplied to the two
sets of coils in the stigmator are varied, corresponding changes
are made to the direction of blur due to astigmatism. The image in
which the diameter of the shadow region is smallest is regarded as
representing the lowest aberration. Under such a condition the
current supplied to the focusing-adjustment coil is varied. The
position of optimal focus is the position at which image lightness
and darkness on the entire subfield are equalized and the diameter
of the shadow region is smallest.
[0074] FIGS. 6 and 7 are plan views of alternative evaluation
patterns 70, 80, respectively, according to a third representative
embodiment. The evaluation pattern 70 of FIG. 6 has a circular
patterned region centrally located in the subfield 71. This
patterned region is divided into sixteen sub-regions 73. In each
sub-region 73, L/S elements are disposed such that they extend
circumferentially. By way of example, the width of a single line or
space is 0.1 .mu.m.
[0075] The evaluation pattern 80 shown in FIG. 7 also is configured
as a circular patterned region centrally located in the subfield
81. This patterned region is subdivided into five concentric,
ring-shaped sub-regions 83. In each sub-region 83, L/S elements are
disposed such that they extend in a radial direction. By way of
example, the width of a line or a space is 0.1 .mu.m.
[0076] The evaluation patterns 70, 80 also can be used to determine
the direction and magnitude of astigmatism by observing the shapes
of shadow regions produced when the patterns are
projection-exposed.
[0077] The embodiments described above were described in the
context of charged-particle-beam (notably electron-beam)
microlithography ("exposure") apparatus. However, the general
principles described above also are applicable to
extreme-ultraviolet (EUV), X-ray, and optical microlithography
apparatus. Also, the reticle is not limited to a transmissive-type
reticle, but alternatively can be a reflective-type reticle.
[0078] Whereas the invention has been described in connection with
multiple representative embodiments, the invention is not limited
to those embodiments. On the contrary, the invention is intended to
encompass all modifications, alternatives, and equivalents as may
be included within the spirit and scope of the invention, as
defined by the appended claims.
* * * * *